The present invention relates to a method of manufacturing distributed or graded index optical elements.
As a method of manufacturing distributed index optical elements, hitherto, there has been known a sol-gel method in which silicon alkoxide Si(OR).sub.4 (R: alkyl radical) is used as the raw material. In a method of obtaining distributed index optical elements by the sol-gel method, silica sol including at least one of metal dopants is prepared, after subjecting the silica sol to a gelling treatment, this wet silica gel is dipped in a solution such as an acid capable of solving the above metal consituent, and then the silica gel is dried and sintered.
As a method of imparting concentration distribution by inducing the above metal constituents as metal alcoxide M(OR')n, an article "r-GRIN Glass Rods Prepared by a Sol-Gel Method" disclosed in Journal of Non-Crystaline Solids 100 (1988) pps. 511-513 is well known. In this method, it is considered that when the sol is prepared and made gelling, the metal constituent (M) is made a bond of silicon (Si) and M--O--Si bond in the gel. The distributed index is produced by the distributed or grated concentration of the metal constituent, so that in order to give distributed index, the concentration distribution of the metal constituent must be obtained by dipping the wet gel into a solution such as an acid.
As a method of inducing the above matal constituent as a metallic salt, there has been well known Journal of Non-Crystalline Solids 85 (1986) pps. 244-246. In this case, it is considered that the method of fixing the metal constituent into the gel and the method of eluting the metal constituent from the gel are different from the case of taking the metal alcoxide in as the metal constituent, and the metal constituent is taken in the gel in the form of metallic salt M.sub.i X.sub.i and thus separated out and deposited in gel pore wall in the form of crystallite. That is,it is considered that the metal constituent (M) does not form a bond of M--O--Si with the silicon (Si) in the gel. In this case, in order to give distributed index, the wet gel is dipped into a solution capable of solving the metallic salt, and again separated out in the gel as crystallite of the metallic salt.
In either the above conventional methods, one of metal dopants other than silicon is used and distributed index optical elements is are obtained by imparting concentration distribution of the metal constituents.
As the conventional method other than the above method, there is known U.S. Pat. No. 4,797,376. This U.S. Patent is a method of manufacturing distributed index optical elements performed by using, in addition to silicon and metal dopants for imparting distributed index, metal dopants for stabilizing glass as third constituent, and having the same function as the previous method using the metal alcoxide.
However, the conventional methods have following problems.
(1) In case of using only one of metal dopants in addition to silicon, difference .DELTA.n of refractive indexes can be controlled by adjusting distributed index imparting conditions, such as amount of metal constituent, concentration of distributed index imparting solution, time of dipping the gel in distributed index imparting solution or the like, but even though difference .DELTA.n of desired refractive index can be obtained, whole refractive index can not be shifted with any width.
(2) In case of using two of metals in addition to silicon, the metal dopants other than the metal dopant having large contribution on refractive index are added to effect glass stabilization but the contribution of these metal dopants on optical characteristics (refractive index and dispersion) of glass is small, resulting in less variations of obtained distributed index optical elements.
(3) In case of using two of metals in addition to silicon, when distributed index imparting conditions are determined so as to make them best for one of metals, the distributed index imparting conditions of the other of metal dopants are determined automatically, for control of the distribution, the shape in variation of concentration distribution of two or more metal dopants can not be obtained.